Organic light emitting diodes (OLED) have been increasingly used as current-type light emitting devices in high-performance display devices. With increasing of display size, conventional passive matrix organic light emitting diodes require a shorter driving time for a single pixel, and thus require an increased transient current, which causes increased power consumption. Meanwhile, the usage of a large current is able to result in that voltage drop on a line of nanometer Indium Tin Oxide (ITO) is too large and that operation voltage of OLED is too high such that efficiency of OLED is decreased. However, the problems can be solved commendably by inputting OLED currents via progressive scanning of switching transistors in active-matrix organic light-emitting diodes (AMOLED).
In the design of an AMOLED backplane, a main problem to be solved is non-uniformity of brightness among pixel unit circuits.
First, for AMOLED, pixel unit circuits are constituted by thin film transistors to supply respective currents for OLED devices. In prior art, Low-temperature poly-silicon thin film transistors (LTPS TFT) or oxide thin film transistor (Oxide TFT) are mostly adopted. Compared to a general amorphous silicon thin film transistor (amorphous-Si TFT), LTPS TFT and Oxide TFT have higher mobility and more stable characteristics, and thus more suitable for applying in the AMOLED display. However, due to limitations of the crystallization process, LTPS TFTs produced on a large-area glass substrate often have non-uniformity on electrical parameters such as threshold voltage, mobility and the like, and such non-uniformity may be converted to current difference and brightness difference among OLED display devices, that is, a mura phenomena appears, and may be perceived by human eyes. Although process of Oxide TFTs has a better uniformity, similar to a-Si TFTs, a threshold voltage of the Oxide TFT may drift under a high temperature or under a case that the Oxide TFT is supplied a voltage for a long time. Due to different display pictures, the amount of the drift of threshold voltage of TFT in respective portions on a panel may be different from each other, which may cause display brightness difference, such display brightness difference often renders an image sticking phenomenon since the display brightness difference has a relation to a previously displayed image.
Second, in large-size display applications, since a power supply line on the backplane has a certain resistance and driving currents of all pixels are supplied from an ARVDD power supply, a supply voltage for an area near to a location of the ARVDD power supply in the backplane is higher than a supply voltage for an area far from the location, such phenomenon is known as a voltage drop of the power supply (IR Drop). Since the voltage of the ARVDD power supply has a relation to currents, IR drop may also cause the current difference among different areas, and thus a mura phenomenon appears during displaying. LTPS process for constructing pixel units by adopting P-type TFTs is sensitive to this problem since a storage capacitor thereof is connected between the ARVDD and a gate of TFT, and thus voltage variation of ARVDD may directly affect the voltage Vgs for driving the gate of TFTs.
Third, the non-uniformity of the electrical characteristics of OLED devices may also be caused during evaporation due to uneven film thickness. For the a-Si or Oxide TFT process constructing pixel units by adopting N-type TFTs, a storage capacitor thereof is connected between a gate of a driving TFT and an anode of OLED, if voltages of anodes of OLEDs of respective pixels are different when a data voltage is transmitted to the gate, the gate voltages Vgs applied actually to TFTs may be different, and thus display brightness difference is caused by different driving currents.
AMOLED may be divided into three categories according to the driving modes: a digital driving mode, a current driving mode and a voltage driving mode. In the digital driving mode, gray scales are achieved by using TFT as a switch to control driving time, and there is no need to compensate the non-uniformity. However, an operation frequency of the digital driving mode is multiplied with increase of the display size, which causes very high power consumption and reaches a physical limit for design in a certain range, thereby being not suitable to the large-size display applications. In the current driving mode, gray scales are achieved by directly supplying currents of different amplitudes to driving transistors, which may better compensate the non-uniformity of TFTs and IR Drop. However, when a low gray scale signal is written, a writing time may be too long since a relative big parasitic capacitor on a data line is charged by a small current. This problem is especially serious in a large-size display and difficult to be overcome. Similar to a driving method for a conventional active matrix liquid crystal display (AMLCD), in the voltage driving mode, a voltage signal representing a gray scale is supplied by a driving IC and is converted to a current signal of a driving transistor in a pixel circuit, such that OLED is driven to achieve the gray scale of brightness. The voltage driving method has advantages of fast in driving speed and easy to implement, and thus is suitable for driving a large-size panel and is widely adopted in the industry. However, as for the voltage driving method, additional TFTs and capacitors should be designed to compensate the non-uniformity of TFTs, IR Drop and the non-uniformity of OLEDs.
There are many pixel structures aiming to compensate the non-uniformity and drift of Vthn and the non-uniformity of OLED, wherein a main design challenge for external compensation is a current sensing circuit, usually each column of pixels in a panel corresponds to a sensing circuit unit respectively in order to increase a reading speed. A main function of the sensing circuit is to convert an output or input current to a voltage signal to be transmitted to subsequent ADC module for further processing. A conventional sensing circuit is constituted by a current integrator, but it cannot make a rapid response when there is a small pixel current.